APPENDIX 277 PREPARATION 53. Aniline.—The reduction of a nitro-compound in an acid solution is a very general method for preparing primary amines. For laboratory purposes it is customary to use tin and hydro- chloric or a solution of stannous chloride crystals (SnCl.>-h2*H.,O) in cone, hydrochloric acid or zinc dust and acetic acid. The manufacture of aniline on the industrial scale is effected by means of iron borings and hydrochloric acid ; but of the latter only a fraction of the theoretical quantity, required by the equation Fe 4- 2HC1 = FeCL, -f H2, is employed. The main reaction is probably represented by the following equation, CJ I5N(X + 2Fe + 4H.20 = C6H5NIL2 + Pea(OH)6. When the base is volatile in steam, as in the present case, the simplest method of separation is to add an excess of alkali and to distil in steam. Otherwise the base maybe separated by shaking out with ether, or the tin may be precipitated in the warm solution by H3S and the filtrate evaporated to dryness. If the compound contains more than one nitro-group, the reduction is carried out with one of the above reducing agents in the manner described, but if it is necessary to reduce only one of the nitre-groups, it is effected by the action of H2S in presence of ammonia (see Prep. 58, p. 154). Another method, which may also be used for determining the number of nitro-groups, is to prepare an alcoholic solution of the nitro- compound, and to add an alcoholic solution of the calculated quantity of stannous chloride. In this way the reduction of the groups may be carried out in succession and estimated. The aromatic amines are colourless liquids or solids, which may be distilled without decomposition. Although they form salts with acids, they are much weaker bases than the aliphatic amines owing to the negative character of the phenyl group. The salts have an acid reaction to litmus, whilst the free bases are neutral. The neutralisation of an aromatic base by acid is usually determined by the use of methyl violet, magenta, or congo-red paper. The first is turned green, the second colour- less, and the third blue by free acid. Aromatic amines, containing the amino-group in the side- chain, have the basic character and properties of aliphatic arrfines.